This invention relates to an optical rotation sensing device which may be used in an advanced global positioning and inertial guidance system.
Optical rotation sensing devices include ring laser gyros, fiber optic rotation sensors, and the like. A fiber optic rotation sensor ordinarily comprises an interferometer which includes a light source, a beam splitter, a detector, and an optical path which is mounted on a platform. Light from the light source is split by the beam splitter into two beams which are directed to opposite ends of the optical path and which then counterpropagate around that path. As the light beams exit the optical path, they are recombined and the resulting combined light beam is sensed by a detector. A sensing circuit connected to the detector determines any phase difference between the counterpropagating light beams.
Assuming that this fiber optic rotation sensor experiences no rotation, ideally no difference in phase between the counterpropagating light beams will be detected. On the other hand, if the sensor experiences rotation, there will be a phase difference between the counterpropagating light beams which can be detected to indicate the extent and direction of rotation.
In a fiber optic rotation sensor, the optical path is provided by an optical fiber which is coiled, usually in multiple layers, around a spool, with each layer containing multiple turns. Currently, such coils are typically wound as quadrupoles. In order to form a quadrupole wound coil, a first end of a continuous optical fiber is wound onto a first intermediate spool, and a second end of the continuous optical fiber is wound onto a second intermediate spool. The first intermediate spool is then used to wind a first layer of turns in a clockwise direction around a sensor spool. This first layer is wound around the sensor spool from a first end to a second end of the sensor spool. The second intermediate spool is then used to wind a second layer of turns in a counterclockwise direction around the sensor spool. This second layer is wound around the sensor spool from the first end to the second end of the sensor spool. The fiber on the second intermediate spool is then wound back from the second end to the first end of the sensor spool to form a third layer. The first intermediate spool is then used to wind a fourth layer of turns from the second end of the spool to the first end.
Thus, a portion of one end of the optical fiber is used to form the first and fourth layers of turns and a portion of the other end is used to form the second and third layers. These four layers of turns are usually referred to as a quadrupole. If "+" and "-" are used to designate the first and second ends of the optical fiber respectively, this quadrupole is wound with a +--+ predetermined winding pattern in which the length of optical fiber in the "+" layers is substantially equal to the length of optical fiber in the "-" layers. The quadrupole is repeated as often as is desired for a fiber optic rotation sensor. Accordingly, if a second quadrupole is wound with +--+ layers about the first quadrupole, the resulting two quadrupole arrangement has a +--++--+ predetermined winding pattern.
When a fiber optic coil wound in this fashion experiences time varying axial and/or radial dependent changes to the optical path, a phase difference between the light beams which counterpropagate around the optical path may result. If so, the fiber optic rotation sensor employing this fiber optic coil produces a false indication of rotation; that is, this phase difference is an error. For example, an axial and/or radial time varying temperature gradient results in axial and/or radial dependent changes to the optical path which produces a phase difference between the counterpropagating light beams, and this phase difference is an error. Errors may be caused by other environmental conditions such as, but not limited to, mechanical and/or acoustic vibration, axial and/or radial time varying pressure gradients, and axial and/or radial time varying strain gradients. Also, winding the layers of the coil inconsistently in the axial and/or radial directions can exacerbate such errors. Thus, although the present invention is discussed in terms of errors produced by axial and/or radial time varying temperature gradients, the present invention is useful in substantially reducing errors resulting from other axial and/or radial influences as well. Consequently, errors resulting from axial and/or radial influences are referred to herein as axial and/or radial related errors.